📝 Document that NotficationManager add* methods return a Notification
📝 Document that NotficationManager add* methods return a Notification

Parts of the Atom ecosystem rely on the fact that these methods return
the Notification that was added. For example:

https://github.com/atom/exception-reporting/blob/4e6866c3a9/lib/reporter.js#L159

Therefore, we should document that these methods are expected to return
the Notification that was added.

brettle/atom,t9md/atom,t9md/atom,liuderchi/atom,andrewleverette/atom,Arcanemagus/atom,liuderchi/atom,PKRoma/atom,Arcanemagus/atom,Mokolea/atom,FIT-CSE2410-A-Bombs/atom,atom/atom,atom/atom,liuderchi/atom,ardeshirj/atom,t9md/atom,brettle/atom,stinsonga/atom,brettle/atom,FIT-CSE2410-A-Bombs/atom,kevinrenaers/atom,stinsonga/atom,atom/atom,ardeshirj/atom,PKRoma/atom,PKRoma/atom,ardeshirj/atom,Arcanemagus/atom,FIT-CSE2410-A-Bombs/atom,andrewleverette/atom,Mokolea/atom,kevinrenaers/atom,liuderchi/atom,Mokolea/atom,andrewleverette/atom,stinsonga/atom,stinsonga/atom,kevinrenaers/atom